Composite materials are used for preparing structural components of airplanes, helicopters, other aircraft and vehicles. A composite material is made from two or more constituent materials: a matrix material and a reinforcement material. The matrix material (also referred to herein as simply the matrix) and reinforcement material (also referred to herein as reinforcements) have significantly different physical properties, chemical properties or both. The reinforcement material imparts its mechanical and physical properties to enhance the matrix material properties resulting in the composite material exhibiting enhanced synergistic properties.
Commonly used reinforcements are glass, carbon, silicon carbide, aramid and a high strength polyethylene fiber (such as Spectra®). The reinforcements may include a variety of fiber types in various forms such as continuous fibers, mat or woven type construction as well as a hybrid of more than one fiber type. Regarding the matrix, there are three main groups of matrices, namely, polymers (also known as plastics or resins), metals (and their alloys) and ceramics. Both thermoplastic and thermoset polymers are employed in making composite materials. Polyethylene, polystyrene, polyamides, nylon, polycarbonates, polysulfaones, and the like are common thermoplastics whereas common thermosets include epoxy, phenolic, polyester, silicone, bismaleimide, polyimide, polybenzimidazole, and the like. The method of production may be selected from RTM (an abbreviation of Resin Transfer Molding) in a closed-mold, compression molding, autoclave processing (open and closed mold), open mold resin infusion (herein abbreviated RI), vacuum bag molding and filament winding of tows or tapes and the like.
Composite materials are used in the preparation of various components for aircrafts due to the considerable reduction in weight achieved in the finished aircraft. Usually each of the various components is manufactured separately and coupled to the structure in a separate procedure until the final structure is obtained. These components are often assembled together by fittings. Coupling of components can be made by welding (for metal parts), by mechanical fastening using rivets or screws (for metals and/or composite parts) or by a combination of adhering and fastening (for metals and/or composite parts). Fittings are usually produced from metal, specifically aluminum or titanium alloys. The fittings are riveted and/or adhered to the composite structure. However, this mode of production results in a higher product price due to the number of steps in the production process. This mode of production is also prone to imprecisions in the final product and mechanical weakening of the assembly. For example, US Patent Application Publication No. 2008/0168619 to Gonzalez et al., entitled “Process for production of aircraft stops, and aircraft door stops made of carbon composite material” describes a process for the production of an aircraft stop that includes a metal insert. The preparation process includes multiple steps. In one step, the metal insert is covered by draping pre-impregnated carbon fiber layers which are eventually folded down on one another and are oriented so as to ensure maximum cohesion of the layers around the metal insert. The metal insert is designed to accommodate a stop screw by which it is secured to the door.
An additional example is shown in US Patent Application Publication No. 2010/0294888 to Texcier et al., entitled “Aircraft opening panel especially an air plane cabin door” which describes a panel made of composite material which comprises a retainer made of titanium. This retainer consists of a retaining part and a mounting part wherein the mounting part is fastened to a beam of the door by means of fasteners, particularly nuts and bolts. In this design the metal fittings and the composite material have different coefficients of thermal expansion (herein abbreviated CTE or CTEs) which yield immense shearing forces that act between the two elements. In order to overcome deterioration of the structure and corrosion in the contact surface between the metal and the composite material, additional procedures of releasing stresses caused by thermal expansion must be added to the production process. Furthermore, modifications introduced in the structure contour impair the aerodynamic shape of the final structure. These are major drawbacks of structures made of a composite material and metal fittings.
Composite structures with fittings made from composite materials exist. In such composites structures, each of the components of the main structure is manufactured individually and then all the components are secured to each other by metal screws and the like. For example, US Patent Application Publication No. 2002/0100840 to Billinger et al., entitled “Device for connecting movable parts with structural elements of airplanes or the like” discloses a connecting device which comprises at least one fitting made of a composite material and designed with an aperture configured to receive at least one bearing. In this publication the attachment between a spoiler (a movable part) and a composite material fitting is effected by gluing the composite material fitting on an indentation formed in an external wall of the spoiler. This particular attachment requires additional connecting elements like screws or rivets.
An additional publication is U.S. Pat. No. 6,234,423 to Hirahara et al., entitled “Composite airfoil structures and their forming methods.” The patent is directed to a box-structure constructed of a composite material upper skin, a composite material lower skin and a spar which is also made of a composite material. Hirahara et al. use laminates of prepreg material for the skins. The spar is made up of flanges which are bonded to the individual skins by an adhesive.
A limitation of the above structure is the manufacturing process which results in a higher product price due to the number of steps in the process and the use of autoclave technology. This mode of production is also prone to imprecisions in the final product since the composite parts are adhered to each other after the curing of each composite component. The glue used for coupling the parts together can modify the dimensions of the product when used in excess and can add extra weight to the final product.